1. Field of the Invention
The present invention relates to an exhaust gas purifying system and an abnormality determining method therefor. Particularly, the present invention relates to an exhaust gas purifying system for purifying exhaust gas of an internal combustion engine mounted on a vehicle, and to a method of determining abnormality thereof.
2. Description of the Background Art
In an exhaust gas purifying system of an internal combustion engine to be mounted on a vehicle, a catalyst is arranged on an exhaust pipe, for purifying exhaust gas. The catalyst is capable of taking and storing an appropriate amount of oxygen, and when the exhaust gas contains unburned component such as hydrocarbon (HC) or carbon monoxide (CO), the catalyst oxidizes such component using the stored oxygen. When the exhaust gas contains oxide such as nitrogen oxide (NOx), the catalyst reduces such substance, and takes and stores the resulting oxygen.
The catalyst arranged on the exhaust pipe purifies the exhaust gas in this manner. Therefore, the purifying capability of the catalyst much depends on the oxygen storage capability. Therefore, deterioration of the purifying capability of the catalyst can be determined by the maximum amount of oxygen that can be taken and stored in the catalyst, that is, the oxygen storage capacity.
Japanese Patent Laying-Open No. 2003-97334 discloses an apparatus for detecting oxygen storage capacity of the catalyst arranged on the exhaust pipe through forced air-fuel ratio control in which the air-fuel ratio of air-fuel mixture supplied to the internal combustion engine, normally adjusted around the stoichiometric air-fuel ratio, is forced to fuel-rich or fuel-lean.
The forced air-fuel ratio control will be described. When the air-fuel ratio is fuel-rich, an exhaust gas with low oxygen, containing unburned component such as HC or CO, is supplied to the catalyst. When such an exhaust gas is supplied, the catalyst emits oxygen that has been stored, to purify the exhaust gas. Therefore, when such a state continues for a long time, the catalyst eventually emits all the oxygen, and reaches a state in which oxidation of HC or CO is no longer possible. In the following, this state will be referred to as a “state of minimum oxygen storage.”
When the air-fuel ratio is fuel-lean, an exhaust gas with excessively high oxygen, containing NOx, is supplied to the catalyst. When such an exhaust gas is supplied, the catalyst takes and stores the excessive oxygen in the exhaust gas, to purify the exhaust gas. Therefore, when such a state continues for a long time, the catalyst eventually stores the oxygen to the full capacity, and reaches a state in which purification of NOx is no longer possible. In the following, this state will be referred to as a “state of maximum oxygen storage.”
In the forced air-fuel ratio control, the air-fuel ratio of the air-fuel mixture, which is normally adjusted around the stoichiometric air-fuel ratio, is controlled such that the state of minimum oxygen storage and the state of maximum oxygen storage are realized repeatedly. The amount of oxygen taken and stored in the catalyst in the process of transition from the state of minimum oxygen storage to the state of maximum oxygen storage, or the amount of oxygen emitted from the catalyst in the process of transition in the opposite direction is accumulated, to find the oxygen storage capacity. Whether the catalyst is normal or deteriorated is determined based on whether the thus found oxygen storage capacity is larger than a prescribed determination value or not.
When the forced air-fuel control described above is to be executed, it is necessary that the internal combustion engine satisfy prescribed operating conditions. By way of example, when stable engine operation conditions can continuously be maintained as in the case of driving down a highway, the air-fuel ratio control can be performed without causing any problem.
When the engine speed or the amount of intake air frequently goes out of a prescribed range as experienced when driving through town, operating condition of the engine is unstable. Therefore, it is disadvantageous to make determination as to whether the catalyst of the exhaust gas purifying system is normal or deteriorated, through forced air-fuel ratio control.
Recently, in some vehicles, fuel is cut during down-hill driving for better mileage. When the forced air-fuel ratio control is conducted while the fuel is cut, the improved mileage would be wasted.
Therefore, while the forced air-fuel ratio control is being done and the prescribed operating conditions come to be no longer satisfied, the forced control is interrupted. Thus, it becomes impossible to complete determination as to whether the catalyst of the exhaust purifying system is deteriorated or not.
Even when the prescribed operating conditions are satisfied again and the forced air-fuel ratio control is done after interruption, whether the prescribed operating conditions are kept for the necessary time period or not is unknown, as the transition from the state of minimum oxygen storage to the state of maximum oxygen storage, or transition from the state of maximum oxygen storage to the state of minimum oxygen storage can take some time, for example, 30 to 40 seconds.
The oxygen storage capacity of the catalyst is determined using an output of an oxygen sensor detecting oxygen in the exhaust gas after passage through the catalyst. If the oxygen sensor fails, whether the catalyst is deteriorated or not cannot be determined accurately. Therefore, it is also necessary to determine a failure of the oxygen sensor as abnormality of the system.